1. Technical Field
The present invention relates to an image forming apparatus that changes the output bias voltage from a charging power supply to change an output amount of bias voltage, based on readings from a rotary attitude sensor to detect whether or not a charger that charges a surface of a latent image carrier assumes a predetermined rotary attitude, and on predetermined change pattern data.
2. Background Art
A conventional image forming apparatus includes a photoconductor as a latent image carrier, a charging roller rotating while contacting the photoconductor, a charging power supply to output bias voltage to be applied to the charging roller, a latent image writing device, and a developing device. An electrostatic latent image is written on a surface of the photoconductor that is uniformly charged by the charging roller, and the electrostatic latent image is then developed by the developing device, thereby obtaining a toner image. In such a structure, the charging roller serving as a charger having variations in the resistance on a peripheral surface of the roller, causes a charged surface of the photoconductor to be varied and image density of the formed image to be varied. In particular, because a gray scale portion such as a highlight portion or a halftone portion tends to be affected by the charged potential of the photoconductor relative to the image density, the aforementioned image density fluctuation worsens.
For the purpose of decreasing the image density fluctuation, the charging roller may be kept away from the photoconductor and allowed to contact a conductive roller. After the charging roller starts rotation, the attitude of the charging roller is detected by a rotary attitude sensor, and if it is detected that the charging roller assumes a predetermined rotary attitude, a predetermined bias voltage is applied to the charging roller. Then, variations in the electrical resistance of one cycle of the charging roller are obtained based on a change in current strength flowing between the charging roller and the conductive roller. After completion of the fluctuation resistance detection, bias change pattern data corresponding to one cycle of the roller responsive to the variations in the resistance is generated. Then, if a print job is started with the charging roller contacting the photoconductor, the photoconductor is charged while changing output from the charging power supply according to the previously-generated bias change pattern data. By reducing the fluctuation of the charged surface of the photoconductor due to the variations in the resistance of the charging roller, image density fluctuation can be prevented from occurring.